The term “high molecular weight polyethylene” is generally used to define polyethylene having a molecular weight of at least 3×105 g/mol as determined by ASTM 4020 and, as used herein is intended to include very-high molecular weight polyethylene or VHMWPE (generally characterized as polyethylene having a molecular weight of at least 1×106 g/mol and less 3×106 g/mol as determined by ASTM 4020) and ultra-high molecular weight polyethylene or UHMWPE (generally characterized as polyethylene having a molecular weight of at least 3×106 g/mol as determined by ASTM 4020).
High molecular weight polyethylenes are valuable engineering plastics, with a unique combination of abrasion resistance, surface lubricity, chemical resistance and impact strength. Thus, in solid, compression molded form, these materials find application in, for example, machine parts, linings, fenders, and orthopedic implants. In sintered porous form, they find application in, for example, filters, aerators and pen nibs.
Currently, high molecular weight polyethylenes are generally produced using Ziegler-Natta catalysts, see, for example, EP186995, DE3833445, EP575840 and U.S. Pat. No. 6,559,249. However, these catalysts have certain limitations with regard to the molecular weight and molecular weight distribution of the polymers that can be produced. There is therefore significant interest in developing alternative catalyst systems for producing high molecular weight polyethylene.
Other known catalysts for olefin polymerization are single site catalysts. According to the present state of technology, high molecular weight polyethylenes are manufactured using these catalysts only in exceptional cases and under economically unprofitable conditions. Thus, with heterogeneous constrained-geometry catalysts, high molecular weight polyethylene is produced only with moderate activity and increased long chain branching, which can lead to reduced hardness and abrasion properties. With so-called phenoxy-imine catalysts, high molecular weight polyethylene is obtained only at low activity at economically disadvantageous temperature levels. Examples of these and other metallocene catalysts are described in WO9719959, WO0155231, Adv. Synth. Catal 2002, 344, 477-493, EP0798306 and EP0643078.
One other potentially useful catalyst system for producing UHMWPE comprises a Group 4 metal complex of a bis(phenolate) ether ligand such as disclosed in International Publications Nos. WO 2003/091262 and WO 2005/108406, the entire disclosures of which are incorporated herein by reference. Research has, however, shown that, although this system provides an effective catalyst for the slurry phase polymerization of UHMWPE with molecular weights unachievable with Ziegler-Natta catalysts, achieving satisfactory catalyst activity is surprisingly dependent on maintaining the process pressure and temperature within narrowly defined limits.
United States Patent Application Publication No. 2008/0051537 discloses a supported, heterogeneous catalyst composition comprising: 1) a substrate comprising a solid, particulate, high surface area, surface modified, inorganic oxide compound, 2) a Group 4 metal complex of a bis(phenolate) ether ligand; and optionally 3) an activating cocatalyst for the metal complex. The catalyst composition is said to be useful for the polymerization of propylene, 2-methyl-4-butene, and mixtures of ethylene with one or more C3-8 α-olefins, especially propylene, 1-butene, 1-hexene, 2-methyl-4-butene, or 1-octene.